There is a continuing need for filters which are capable of removing finer contaminants while not increasing the flow resistance and limiting dirt capacity. This need is nowhere more pronounced than in magnetic recording technology. Magnetic recording technology is steadily moving in the direction of greater record density on devices such as computer memory disks and magnetic tape. The manufacturing processes for devices of these higher densities require finer filtration devices while not adversely affecting current filter flow resistance and dirt capacity values.
Magnetic recording devices are manufactured by coating a substrate with a layer of fine particles which can be magnetized by the recording device to preserve certain information. To deposit the layer of particles, a mix of adhesive, solvent, and magnetic particles is applied to the surface of the substrate. The coating mix typically contains aromatic solvents, such as THF, toluene, MEK, and cyclohexane, and are highly viscous. In manufacturing higher density recording devices, the particles must be smaller, and the deposited particle layer must be very uniform.
To achieve a uniform particle layer, it is essential that all impurities, gels, agglomerates, and the like be removed from the coating mix before application to the substrate. The removal is typically accomplished by passing the coating mix through a filter immediately prior to applying it to the substrate. The characteristics of the filter must be such that all contaminates are removed, but the passage of the coating particles is maximized. For economical manufacturing, the rate at which the magnetic coating mix will flow through the filter under a given propelling pressure, or the resistance to flow at a given flow rate, and the volume of coating mix which can be filtered before the filter plugs with contaminants are important filter performance parameters.
For general use magnetic video tape, for example, filters rated for removal of contaminants larger than 5-10 micrometers are normally used for final filtration of the magnetic coating mix. Such filter elements are typically cylindrical depth filters constructed of wound fibers or melt-blown filaments. Filters constructed of pleated paper of plastic fibers, such as polypropylene, are also used in the industry. For more recent magnetic recording devices such as high density memory disks and high band video tape, final filters rated for removal of particulates of 5 micrometers or smaller are required. Cylindrical filters with finer removal ratings can be used, but they have a correspondingly higher resistance to flow, and, consequently, more filters arranged in parallel are required to permit proper operation of batch coating processes.
Accordingly, a filter is desired which is capable of finer filtration while having a low resistance to flow and a high dirt capacity. Such a filter should preferably be capable of filtering contaminants from magnetic coating mixes of approximately 5 micrometers or less and, most preferably, less than approximately 3 micrometers.